Some 20,000 km of volcanic arcs, roughly one-third the length of the global mid-ocean ridge (MOR) system, rim the western Pacific Ocean. Compared to 25 years of hydrothermal investigations along MORs, exploration of similar activity on the estimated ∼600 submarine arc volcanoes is only beginning [Ishibashi and Urabe, 1995; De Ronde et al, 2003] . To help alleviate this under-sampling, the R/V T G Thompson in early 2003 (9 February to 5 March) conducted the first complete survey of hydrothermal activity along 1200 km of the Mariana intra-oceanic volcanic arc. This region includes both the Territory of Guam and the Commonwealth of the Northern Mariana Islands.